Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle- or large-sized devices, such as vehicles, use a middle- or large-sized battery module having a plurality of battery cells electrically connected to each other because high power and large capacity are necessary for the middle- or large-sized devices.
Preferably, the middle- or large-sized battery module is manufactured with small size and weight if possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle- or large-sized battery module. Especially, much interest is currently generated in the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the weight of the pouch-shaped battery is small, the manufacturing costs of the pouch-shaped battery are low, and it is easy to modify the shape of the pouch-shaped battery.
For the middle- or large-sized battery module to provide power and capacity required by a predetermined apparatus or device, it is necessary for the middle- or large-sized battery module to be constructed in a structure in which a plurality of battery cells are electrically connected in series to each other, and the battery cells are stable against an external force.
Also, the battery cells constituting the middle- or large-sized battery module are secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from the high-power, large-capacity secondary battery during the charge and discharge of the battery cells. If the heat, generated from the unit cells during the charge and discharge of the unit cells, is not effectively removed, the heat accumulates in the respective unit cells, with the result that the deterioration of the unit cells is accelerated. According to circumstances, the unit cells may catch fire or explode. For this reason, a battery pack for vehicles, which is a high-power, large-capacity battery, needs a cooling system for cooling battery cells of the battery pack.
In relation to this matter, some conventional arts disclose middle- or large-sized battery modules constructed in structures to improve cooling efficiency. For example, Japanese Registered Patent No. 3355958 discloses a middle- or large-sized battery module including a plurality of battery cells stacked in a limited space, such as a vehicle, constructed in a structure in which protrusions defining a coolant flow channel are formed at the outside of each battery cell, such that a coolant cools the battery cells while the coolant passes through each stacked battery group after the coolant is introduced between the battery groups, in order to prevent the cooling efficiency of the coolant from lowering when the coolant passes through one stacked battery group in a state in which the temperature of the coolant increases after the coolant passes through another stacked battery group. Also, the battery cells used in this patent are alkaline batteries such as nickel-metal hydride batteries. For this reason, it is required to form the external shape of the battery module such that the battery module itself exhibits high mechanical strength. Consequently, plates are disposed at opposite sides of the stack of the battery cells, and the plates are fixed by bands, to construct the middle- or large-sized battery module.
As a result, the disclosed middle- or large-sized battery module has a problem in that the volume and weight of the battery module greatly increase due to its external shape to provide high mechanical strength although the cooling efficiency of the battery module is partially improved.
In a middle- or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the nonuniformity of the performance is the nonuniformity of cooling between the battery cells. For this reason, it is required to provide a structure to secure the uniformity of cooling during the flow of a coolant.
The cooling efficiency greatly changes according to the structure of a coolant flow channel in the battery pack. Consequently, there is a high necessity for a middle- or large-sized battery pack that is capable of preventing the leakage of a coolant to the outside, while the middle- or large-sized battery pack is constructed in a more compact and stable structure, thereby improving the cooling efficiency.